Method of selective filtering of frequency band ranges



Sept. 29, 1942. BENDEL ETAL 2,297,451 7 METHOD OF SELECTIVE FILTERING OFFREQUENCY BAND RANGES Filed May 4, 1940 E 2 Sheets-Shea, 1

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v LEW-.3 1 b 440 l L C 2 L -LT d I (q 440 L 280 0 KI LOCYCLES PER SECOND20 1 10 190 abo 4'20 69'0 mocmss PER ssco/vo HANS JA COBY ATTORNEYPatented Sept. 29, 19 42 METHOD or sELEc'rivs m'rsnmc F FRE- QUENCY BANDRANGES Hermann Bendel, Berlin-Charlottenburg, Karl- HeinrichKrambeer,Berlin-Spandau, and Hans Jacoby; Finkenkrug, Kreis Osthavelland,Germany; vested in the'Alien Property Custodian Application May 4, 1940,Serial No. 333,372 In Germany and Japan-December29, 1938 a Claims. (01.178-44) It is often necessary to filter out selectively to. partial bandfrom a frequency band and to block I it selectively. As long as this isdone in a range ment. In the case of higher frequencies a veryconsiderable amount of filter means is required. If, furthermore, aparticularly sharp cutoff of the fianks of the curves is necessary, thiscan no longer be attained by means of coils and condensers or even withthe use of crystal filters, In multiple carrier frequency systemscomparatively wide unused frequency gaps are, therefore, requiredbetween the message bands or groups of message bands.

In order to achieve an unbroken continuity of message bands in carrierfrequency message communication systems, methods are already known andsystems constructed according to which, with the utilization of amultiple modulation, shift the required selectivity substantially into arange of lower frequency in order thus to permit the realization ofrequired filters. However, the known methods are not directly suited forfiltering out of a frequency band a wide partial band, such as forinstance a. television band, or one group or several groups of adjacentmessage bands such as will be necessary for instance in the case of atransmission system having more than two terminal exchanges. Forinstance, at an exchange it is necessary to filter out a televisionband, or one or several groups of message bands in a selective mannerand then to fill out again the open frequency ranges which were occupiedby the removed bands. When resorting to the known measures for thispurpose, an excessively large amount of apparatus is required forfiltering out a wide partial band, such a partial band which may consistof, for instance, one or more .groups, each having ten message bands,must be' partitioned to form a. multiplicity of part channels after theconversion into the low frequency range,' in order that throughselective means in the audio-frequency range a sufficiently sharpseparation from the adjacent frequencies can be obtained. The sameamount of equipment is necessary with a frequency conversion into ahigher frequency range where the filtered out partial band is usedagain, for instance, to fill out a second wide band cable.

However, with this large amount of structure, which is practically thatneeded in the terminal exchanges for transmission and filtering of apartial band of same width, it has not been possible, as far as we areaware, to again render the frequency band range completely available bythe filtering out of the partial band. The liberated range cannot bemade completely available for re-use simply through customary meansutilized for the blocking of a frequency band range, so that largerfrequency gaps must be taken into account.

The method according to the present invention offers a substantialreduction in means required as well as the possibility of covering afrequency band having practically no gaps.

For the selective filtering and for the selective blocking of a wideband contained within the said portion of the frequency spectrum, theinvention contemplates the shifting of each edge of the partial band, tobe filtered out and to be blocked, individually through conversion intoa sufiicientiy low frequency range and the carrying out of theexactseparation therein. The conversion of the frequency edges is, by ourinvention, carried out through modulator arrangements such that for oneedge separation, the frequency sequence is retained and for the otherthe sequence is reversed whereby the region of sharp cutofi to beattained always lies at the bottom in the frequency range.

When filtering a wide partial band it is seen that the latter isobtained with a simple selective filter and that for obtaining an exactseparation or cut-off, of the undesirable neighboring frequencies saidband is converted into a sufiiciently low frequency range whileretaining the frequency sequence for cutting off one edge and reversingthe frequency sequence for cutting the other edge, whereby after eachconversion an exact separation is carried out at the lower frequencyedge of the converted band.

For the selective blocking of a wide. partial band, the adjacentfrequency band ranges are separated with a simple selective filter andeach partial band thus obtained is converted into a sufficiently lowfrequency range, the lower one with reversing of the frequency sequenceand the upper one without such reversal. At the lower frequency edge ofthe two converted partial bands back into their initial position.

The invention will be elucidated in the following description withreference to the Figures 1 and 2 representing schematically examples ofapplication of the method according to the invention, while Figures 3and 4 illustrate in block diagram' form circuit arrangements which maybe used for practicing thegexamples illustrated in Figures 1 and 2.

In Figure 1 the various steps of the method are shown in the portions ofthe figure labeled a to f inclusive. From the frequency'band Iextending, for instance, from 190 kilocycles per second up to 690kilocycles per second, the message band group 11 indicated in dash lineand which is to reach from 300 kilocycles per second up to 420kilocycles per second is to be filtered out. In accordance with theinvention this is done in the following manner:

(a) The group is filtered out with a band-pass filter having a lesssharp cut-ofi' than finally required.

(-b) There is carried out with a carrier frequency T1 a frequencyconversion into a convenient low frequency range, for instance,extending from 20 kilocycles per second to 140 kilocycles per second.

(c) There is carried out through selective filtering means an accurateseparation of the undesirable neighboring frequencies at the lowerfrequency edge.

(d) The carrier or conversion oscillator frequency T2, which may bechosen equal to the carrier frequency T1, is employed to carry out afrequency conversion to a higher frequency range, which may or may notbe the same as the original range.

(2) With theconversion oscillator frequency T3 a renewed frequencyconversion to the low frequency range is carried out. The carrierfrequency T3 is to be chosen in such a manner that the frequencysequence of the group is reversed relative to that of the firstfrequency conversion (12). The choice of T3 is such to advantage thatthe position in the frequency spectrum, after the frequency conversion,is the same as in the case of the first frequency conversion (b).

(I) The exact separation of the undesirable (a) The partial band Ia isconverted into the original frequencyrange by means of the con- 7version frequency T1.

The upper part Ib of the frequency band 'is dealt with in the same way,i. e. this part is converted with a conversion frequency T: which mayquency edge of this band, the conversion into neighboring frequencieswhich are now situated at the bottom is carried out, as explained instep c.

The frequency band group thus obtained shows sharp frequency edges andthrough renewed conversion, step (a), with a conversion frequency may beshifted to a desired range. For instance, it can be shifted to a rangecovered by another wide band cable.

For again utilizing the liberated frequency band it is necessary toprovide a sharply selective blocking for this range. Figure'2 shows theway in which this can be done in accordance with the invention:

(a) The entire frequency range is divided up into two parts Ia and II)by means of a coil and condenser connection, or other cross over filterwhose cross point is situated in the range II which is to be blocked.

(b) The lower part Ia of the frequency band is converted by means of theconversion frequency T1 for narrow blocking ranges this may liesubstantially exactly in the center of the range to be blocked. Theconversion may be such that the frequency edge F1 is shifted into aconveniently low range as shown by F1.

(0) The undesired neighboring frequencies at the lower frequency edgeare accurately sepa rated.

the original position is carried out with the same conversion frequency.Then a frequency band is obtained whichjs .utilized to the frequency F1,is free from the frequency F1 tqF: and again utilized above thefrequency F2.

In Figure 3 is shown in block diagram form an organization forpracticing the method of Figure 1. The input signal, as indicated by thearrow labeled input, is applied to band pass filter 30 wherein step a ofFigure 1 takes place. The selective frequency band F1, F2 is thenapplied to mixer 3| wherein it is mixed with oscillations generated atoscillator 32 thus performing step b of Figure 1. Sharp out high passfilter 33 accurately separates the lower edge frequencies of theconverted band as at step c of Figure 1 and then in mixer 34 the band isagain converted to its original position as in step d of Figure 1. Then,in mixer 35 the band ofcfrequencies F1, F2, now accurately cut at oneedge, is mixed with oscillations from oscillator 36 of such frequencythat inversion of the band takes place, thus performing step e of themethod of Figure l. The converted inverted band is passed through sharpout high pass filter 31 and then applied to mixer 38, thus reconvertingthe band to its original position in the frequency spectrum. The bandnow has both edges accurately defined and may be usedjn the mannerrequired.

Figure 4 is similar to Figure 3, but illustrates an organization forpracticing the method of Figure 2 wherein a sharply selective cutting ofa predetermined range in a frequency spectrum is accomplished. The inputis applied to crossover filter 40 where the spectrum is divided into twobands of frequencies I11, Ib, thus perfroming step A of Figure 2.- Thefrequency band Ia is applied to mixer II where it is mixed withoscillations from oscillator 42, thus performing step b of Figure 2.

It will be noted that the frequency band is converted to a lowerfrequency and inverted. An accurate separation of the lower edge of theband at F1 is then accomplished in sharp cut high pass filter 43, asindicated in step c of Figure 2. The converted band of frequencies Inwith the lower edge accurately defined is then applied to the mixer 44where it is reinverted and converted to its original position in thefrequency spectrum, as indicated in step d of Figure 2. The frequencyband Ib is similarly dealt with in the lower portion of Figure 4, themixers II and M and the high pass filter 43' acting similarly as justabove described. The oscillator 42' generates a frequency such thatinversion does not take .place in the conversion in mixers 4| and H. Thecombined output from mixers 44 and 44' provides a frequency band whichis utilized up to frequency F1 of Figure 2, is free from the frequencyF1 to F1 and again utilized above the frequency F2.

We claim: 1

1. The method of separatinga desired band from a range of frequencieswhich comprises filtering out a band wider than and including saiddesired band, converting said wider band to a low frequency range,inverting and converting said wider band to said low frequencyrange,sharply discriminating in each of said converted bands against theundesired low frequency portion thereof and reconverting said convertedbands to a predetermined single position in said range of frequencies.

2. The method of separating a desired band from a range of frequencieswhich comprises filtering out a band wider than the desired bandconverting said wider band to a low frequency range, sharplydiscriminating against the undesired portion of the low frequency end ofsaid wider band, converting and inverting said wider band to said lowfrequency range whereby the remaining undesired portion of said band isat the low frequency end discriminating sharply against said remainingportion and convertin said desired band to a predetermined position insaid range of frequencies.

3. Method of filtering out selectively a wide partial band of afrequency band, characterized in that the partial band is filtered outand that for separating accurately the undesirable neighboringfrequencies said band is converted into a suffioiently low frequencyrange on the one hand, with retaining of the frequency sequence and onthe other hand with reversal of the frequency sequence, whereby aftereach conversion there is carried out an accurate separating of therespective lower frequency edge of the converted band.

4. Method of blocking selectively a wide partial band in a; frequencyband, characterized in that the adjacent frequency band ranges areseparated and that each partial band thus obtained is converted into asufiiciently low frequency range, the lower one with reversal of thefrequency sequence and the upper one without reversal of the frequencysequence and that a sharp discrimination is carried out at the lowerfrequency edge only of each of the two converted partial bandswhe'reafter the two partial bands are returned again into their initialposition.

5. A method according to claim 4 characterized in that identical carrierfrequencies are employed respectively for the conversion into the lowfrequency range and for the conversion into the higher frequency range.

6. Method according to claim 3, characterized in that identical carrierfrequencies are employed respectively for the conversion into'the lowfrequency range and for the conversion into the higher frequency range.

'7. In a communication system, the method of spectrum, sharply definingthe edges of said band to be substituted by filtering from a range ofsiging the steps of separating said spectrum into two partial bands, oneabove and one below said band to be blocked, each partial band includina portion of said blocked band, converting each partial band into a lowfrequency range, the lower of said partial bands with frequencyinversion, sharply discriminating against frequencies below the lowfrequency edge of each of said converted partial bands and reconvertingsaid partial bands to their initial position in said spectrum.

HANS JACOBY.

HERMANN BENDEL.

KARL-HEINRICH KRAMBEER.

